This invention relates to a real time adaptive control and quality assurance system for a resistance spot welder, and to a method of controlling the welding process.
During the weld operation, variations in the process operating point occur from weld to weld due to several conditions such as: material surface variations, variations in workpiece geometry, flattening of electrodes, machine malfunction, operator error, etc. These variations are often impossible for the operator to detect and cause welds of different quality to be produced.
Factory resistance spot welding facilities employ several costly procedures in an attempt to overcome the uncertainty associated with this metal joining procedure. These include making two to three times the number of welds that would be required if all welds were known to be good, the periodic destructive testing of coupon test samples which are supposed to be representative of the actual welding conditions, and brazing of each weld. These quality control procedures are laborious and have a severe impact on factory productivity as well as making the spot welding procedure more complicated, and do not ensure complete reliability of the welding process.
It was therefore necessary to develop a system for resistance spot welding which will provide a new level of reliability and confidence for this basic joining process. The ramifications of allieviating the problem include eliminating destructive testing and extra welds, doing away with the brazing process, in the case of aircraft, shaves a few pounds off engine weight and hundreds of pounds off the total aircraft weight. The complete removal of guess work associated with the spot welding process eliminates the need for a trained operator to control the machine. This makes the invention suitable for applications where the workpieces to be welded are automatically positioned and controlled by robot mechanisms. In addition to the vast increase in productivity which will be realized, resistance spot welding could be employed in place of other joining techniques where more costly procedures such as riveting are utilized. This would greatly broaden the spectrum of applications for resistance spot welding.
Most resistance spot welding machines employ no feedback during the weld process. The operator sets the weld controls, which are established by trial and error, for a given machine/electrode/workpiece configuration. These settings are unique for each machine since they are not defined in terms of calibrated physical properties, performing the same job on a different machine requires new control settings to be established by trial and error. Any change in the operating point of the machine or change in workpiece properties will result in a faulty weld.
There are essentially four types of spot welding monitors/controls on the market which are grouped according to the single parameter measured in the welding process. They are: (1) thermal expansion monitors, (2) nugget resistance monitors, (3) power input (weld energy) monitors, and (4) ultrasonic, acoustic, infrared type monitors. Some of the more recently designed microprocessor-based weld controllers do implement single variable feedback algorithms and are capable of producing good results under certain restricted conditions. The algorithms are limited in complexity and provided compensation only once per weld cycle. These units are difficult to program and require a knowledge of systems theory in order to assure process stability.
The major serious flaw with these units is that no diagnostics are performed along with the in-process feedback algorithm to insure that any change in machine or workpiece characteristics are within sufficient limits to be effectively compensated for by the algorithm. As a result, poor welds can be produced by these systems without being detected. When the process does abort due to an inability of the feedback control to fall within predefined limits, no diagnostic information is provided for the operator describing the nature of the problem. This task is left to the subjective judgment of the operator.